A product is any item or service that your organization sells.
To identify products of interest to a contact or account, add products to the opportunity records. Microsoft Dynamics CRM calculates the estimated revenue of the opportunities that have associated products. This is based on the product's base price, volume discount, manual discount, and other pricing modifications.
When you are ready to start a sales transaction, create quotes, orders, or invoices from within the opportunity record that has associated products. Starting with the opportunity tracks the overall sales process and expected revenue. If you do not start a sales transaction from within an opportunity record, add an existing product to quotes, orders, or invoices separately.
Associate the products created in your product catalog with other records in Microsoft Dynamics CRM to simplify your sales process. This gives you the ability to recalculate your quote, order, or invoice values based on any changes made to the products.
This shows the ways that you can use products in Microsoft Dynamics CRM to simplify your sales process.

Related Links
Work with Opportunities
Work with Quotes
Work with Orders
Work with Invoices
Overview: Build the product catalog